#define H_MIN 60. /* Minutes per hour. */
#define MIN_S 60. /* Seconds per minute. */
#define WEEK_DAY 7. /* Days per week. */
+#define WEEK_S (WEEK_DAY * DAY_S) /* Seconds per week. */
extern const struct substring empty_string;
double expr_wkyr_to_date (double wk, double yr);
double expr_yrday_to_date (double yr, double day);
double expr_yrmoda (double year, double month, double day);
+double expr_date_difference (double date1, double date2,
+ struct substring unit);
+double expr_date_sum (double date, double quantity, struct substring unit_name,
+ struct substring method_name);
struct substring alloc_string (struct expression *, size_t length);
struct substring copy_string (struct expression *,